The 400 was Chrysler's revised B-engine for 1972, replacing the 383 that had run through 1971. Same B-block architecture but with a larger 4.342-inch bore (vs the 383's 4.25"); same 3.375-inch stroke. Designed to meet 1972's revised emissions standards while keeping the displacement-per-horsepower curve up where Chrysler wanted it.
Two passenger-car variants: M-code 400 2-bbl (lower compression for emissions compliance) and P-code 400 4-bbl Magnum (the performance version, often paired with the Magnum NOM nodular crank and four-barrel induction).
Production 1972–1978. The 400 fell off as the muscle market collapsed and Chrysler shifted to smaller-displacement engines + the Magnum 360 for the late-'70s fuel-crisis era.